This invention relates to battery testing and sorting. More particularly, it concerns a method for testing individual batteries for anticipated shelf-life and culling those individual batteries which are not acceptable.
In the merchandising of most battery powered appliances and the like, it is accepted practice to separately package and retail the batteries required for use of the appliance. Among the reasons for this practice are a different manufacturing origin of the appliance and the batteries, different storing, shipping and handling requirements of the appliance and battery, and possible damage to the appliance by defective batteries. Most germane to these reasons and others for separate merchandising of batteries is that the electric charge stored by substantially all batteries deteriorates in time, without use, so that the batteries exhibit a shelf-life of limited duration whereas the appliance with which they are used will last indefinitely without use if proper storage conditions are met.
An exception to the practice of separately merchandising batteries and related goods is exemplified by the merchandising of photographic film for use in electrically powered cameras. Film packs for use in instant cameras of the type available from Polaroid Corporation, Cambridge, Mass. under the trade designation "POLAROID SX-70 LAND FILM", for example, include a sheet-like battery arranged to be engaged by camera supported contacts upon insertion of the film pack into the camera, thus assuring that the camera powering battery is replaced after exposure and motorized processing of the limited number of film units supplied with each film pack.
The sheet-like batteries used in such film packs employ materials selected to require storage conditions which correspond ideally with those of the photographic film units included in the film pack. Also, the construction of such batteries has been developed to a point where the open circuit voltage decay rate is exceptionally low. In this latter respect, the disclosure of U.S. Pat. No. 4,028,479 is illustrative. In spite of the complementary storage condition requirements and construction of this type of battery, however, the shelf-life characteristics of a given battery are difficult to discern at the time of battery production and often will have a longevity falling below that of the corresponding shelf-life of the film units with which they are packaged.
To minimize film wastage as a result of abnormally short battery shelf-life, current procedures used in the manufacture and testing of such batteries involve a batch or production run sampling technique by which the anticipated shelf-life of each batch or run of batteries is ascertained before any of the individual batteries in such a batch or run are assembled and packaged with film units in a film pack. Specifically, a sampling of batteries manufactured in each production run or batch is tested immediately upon completion for the voltage of the stored electrical charge, and the voltage of the sample is recorded. Samples are then stored for a period of time, retested for voltage charge, the retested voltage compared with the original test voltage and the voltage decay rate computed to provide a measure of shelf-life. If the shelf-life of the sampling of any batch indicates a shelf-life shorter than is acceptable, the entire batch or production run corresponding to the sampling is discarded to insure that only those batches or production runs of batteries exhibiting an acceptable shelf-life will be used.
While the present procedures have demonstrated statistical soundness, it has been found that the shelf-life of individual batteries in a given batch or production run may vary considerably. As a result, many of the batteries discarded as a result of the batch or production run sampling technique are found to exhibit an acceptable shelf-life. Accordingly, current procedures are in need of improvement from the standpoint of reducing battery wastage.